FAQ

ALIGNMENT ACCURACY

Is the use of laser alignment equipment more accurate than dial indicator alignment?

Before starting the actual alignment procedure, the right alignment tool for the measurement must be chosen. Laser alignment equipment must be used where both shafts can be rotated and where e.g. an elastic coupling is mounted. Because of the difficult turning of an uncoupled propeller shaft, the use of dial indicator reading is far more convenient and accurate in this situation. Special attention must be paid to bar sag in the dial indicator measurement. A bracket made from angle bar is 'not done' either because of the bar sag characteristics while turning. Always use T, square or round tube bar material.

Is rotating equipment aligned within a target of 0.05 mm?

The most efficient way of energy transport is when the centrelines of the two shafts are co-linear. In theory, this means that a 'zero' alignment must be present under the operational conditions of the installation. During alignment, parameters such as thermal growth, bending of shafts, tilting of shafts, dynamic movement of shafts, deformation of elastic coupling and top plate (hull) deformation must be taken into account. A 0.05 mm tolerance on the alignment target is quite common for all rotating equipment in the Marine Industry. Slow shaft speeds can handle more misalignment than, for example, generators running at 1800 rpm. An alignment target or what is commonly referred to as a gap and sag alignment target with an alignment tolerance must be calculated in cold static (open shaft) conditions.

The elastic coupling can adopt a large misalignment. Do I still have to align my equipment within 0.05 mm?

At a basic level, the elastic coupling is designed to dampen torsional vibrations from an engine. Elastic couplings can handle some misalignment but resultant forces may be absorbed by the bearings of the adjacent rotating equipment resulting in possible vibrations, heat-up and accelerated wear.

ALIGNMENT METHOD

How do I align cardan shafts?

Cardan shafts are used in two configurations or a combination of these two. One is the parallel offset configuration, or what is commonly referred to as a Z drive and the other is the angle gap configuration or what is commonly referred to as the W drive. Drives with cardan shafts can only be aligned after the cardan shaft itself has been removed. De Z drive is aligned using laser alignment equipment in combination with specially designed brackets. The W drive is aligned using equal length dummy pins on both couple flanges. The W drive is the most common drive to align than the Z drive. It is also far more precise. When the sharp points of the
dummy pins are within one mm from each other, the cardan shaft homokinetic coupling angle difference will be negligible.

Is the gap and sag value the leading factor in alignment?

No, gap and sag alignment values have to be seen as a 'pre-alignment' procedure. Because shipyards around the world are using the dial indicator as an alignment tool, all calculations regarding alignment have a gap and sag value as a result. Alignment must always be verified using a jack load measurement test or strain gauge bending stress measurement. When theoretical jack load values meet the actual measured values, alignment is within the specifications.

Must we disconnect couple flanges to check the alignment?

No, with an elastic coupling between two machines, the laser alignment technique is used to check the alignment. In propulsion shafts, the strain gauge technique is used for measuring the bending stress in vertical and horizontal plane. The measured bending stress is used as input in an FEM (Finite Element Model) computer program to determine the theoretical bearing load, gap and sag, etc., in both the vertical and horizontal planes. The calculated jack load values are verified against the actual measured jack load values with the use of a hydraulic jack and electronic load cell.

ALIGNMENT TARGET DETERMINATION

How is the gap and sag alignment target calculated?

The thermal growth is calculated using the equation for the expansion of steel. The tilting of shafts is calculated by using the clearances and distances of the bearings. The dynamic movement of shafts and the deformation of elastic coupling are difficult to calculate. It is easier to perform a dynamic measurement under operational conditions by using what is commonly referred to as Eddy Current Probes or Proximity Probes. The bending of shafts is calculated by using an FEM calculation program.

Are bearing bushes of propulsion shafts always placed in a straight line?

This is one possible solution. Instead of placing the shaft on the bearings, the bearing must be placed on the shaft. A propeller shaft will hog because of, for example, the weight of the propeller on the shaft. When an in-line bearing is placed i.w.o. the hogging, there will be no bearing - shaft contact and, therefore, there will be no load on the bearing. By placing this bearing with a certain offset (placing the bearing on the shaft) bearing, shaft contact and, therefore, a load will again be established. When an FEM computer program is used, the complete propulsion line is checked on bearing load, bearing offset, bearing slope mismatch, bending stress in shafts, shear force in shafts, gap and sag values and bearing load verification with jack load values under different operational conditions such as cold, hot and fully loaded vessel.

How is the hull deformation calculated?

By using the same strain gauges and FEM computer program, bending stress of the propulsion shaft is measured under different loading conditions such as cold ballast condition and cold fully loaded condition. A difference in bending stress is only caused by hull deformation and, therefore, a different position of the gearbox/two-stroke engine because of draft differences. The difference of these two conditions is the relative hull deformation between the stern tube and gearbox or two-stroke engine. The optimum bearing offset can be calculated suitable for all investigated operational conditions with the results of the measurements.

CHOCKING METHODS

Which type of installation solution do I have to use for my rotating equipment?

Which is the right chock for the installation equipment will depend on where the equipment will be used. For example, solid steel chocks can be used in heavy industry such as with regard to sand dredgers and stone cutters for easy removal during (frequent) repair of the equipment. Epoxy resin and adjustable steel chocks are frequently used on-board push boats and cargo vessels because of the quick and short installation time. On-board luxury motor yachts where sound and vibration is the most important factor, resilient mounts under gearboxes and engines are frequently used.